1. Field of the Invention
The present invention relates to a touch panel device for detecting a touch position of an object such as a finger by detecting a position where a surface acoustic wave is attenuated due to a touch of the object, and a control method and a control device for eliminating noises in the detection.
2. Description of the Prior Art
Such a touch panel device is often used as an input device of a personal computer, a mobile computer or a portable information terminal (or a personal digital assistant, PDA). A user of the touch panel device can enter information by touching a display screen of a display device with a finger or a pen.
There is one type of the touch panel device, which utilizes a surface acoustic wave (SAW). This includes transducers arranged at sides of a touch area for exciting a surface acoustic wave or receiving the same. When a finger or the like touches the touch area, the touch position is detected based on a position where the surface acoustic wave is attenuated. The applicant has proposed one type of such touch panel devices in Japanese unexamined patent publication No. 2004-171213. This touch panel device uses transducers having an electrode structure in which a piezo-electric thin film is sandwiched between a comb electrode and a plate electrode so that only one electrode is arranged on one face (a single phase transducer, SPT) and has a chevron-shaped electrode structure in which plural V-shaped comb electrodes are arranged successively.
The touch panel device includes a rectangular transparent substrate and four transducers disposed on the substrate. Two of the four transducers are for excitation arranged at the upper and the lower sides of the substrate, and the other two are transducers for reception arranged at the right and the left sides of the same. The portion surrounded by the four transducers is the touch area.
An excitation voltage of a burst wave is alternately applied to the transducers arranged at the upper and the lower sides so as to generate a surface acoustic wave. The generated surface acoustic wave propagates on the substrate in a diagonal direction so that the transducers arranged at the right and the left sides can receive the same. The surface acoustic wave propagates in parallel with the diagonal line. Therefore, the closer to the diagonal line, the longer a propagation distance as well as a time period until arrival is. Accordingly, the transducer for reception produces a sequential reception signal (time domain waveform) due to one burst wave.
When a finger, a pen or the like touches a position in the touch area, the surface acoustic wave is attenuated at the touched position. Responding to it a level of the reception signal drops, so the position touched by the finger or the like can be detected base on the position where the level drops.
In order to detect the drop of the level of the reception signal (the time domain waveform), a difference between the signal and a time domain waveform (slice data) stored in a memory in advance is determined. If there is a drop of the level due to a touch of a finger or the like, the difference increases. When this difference exceeds a threshold value (a slice level), it is decided that a finger or the like touched. If the slice level is set to a smaller value, a small value of the difference can be detected. In other words, detection sensitivity (touch sensitivity) increases.
A pressure (weight) of contact with a finger or the like is substantially proportional to an attenuation level of the surface acoustic wave. It is preferable to use a slice level as small as possible in order to detect a small pressure applied by a finger or the like.
As described above, it is preferable to set the slice level as small as possible for improving detection sensitivity. However, it is found that there is a possibility of fluctuation generated by a dirty surface of the touch area or noises of incoming radio wave, and the fluctuation is not negligible to the detection sensitivity of the received time domain waveform. In other words, if the detection sensitivity is increased, the difference generated by the fluctuation due to the noises or the like may exceed the threshold value, resulting in an error of detection.
As a countermeasure against this problem, it is possible to add a smoothing process on the received time domain waveform so as to reduce the fluctuation. Thus, the influence of the noises or the like can be reduced substantially. However, once a noise like a spike noise having a large variation is picked up, it is very difficult to eliminate such a noise, which may cause an error of detection.
In particular, as cellular phones have become widespread recent years, influences of noises due to microwave radio waves emitted by the cellular phones have become non-negligible.